The epoxidation of olefins utilizing hydrogen peroxide as the oxidant is a widely practiced method to make epoxy compounds. In recent years, rhenium complexes have been reported as effective catalysts with hydrogen peroxide. However, this procedure has been shown to require anhydrous hydrogen peroxide. Hydrogen peroxide is commercially available only in the form of aqueous solutions, and aqueous conditions tend to favor further reaction of the epoxide to give the diol. Thus, the procedures in the open literature in which rhenium (VII) is used take steps to dry the hydrogen peroxide solution in an alcohol prior to its use.
For example, U.S. Pat. No. 5,155,247 issued on Oct. 13, 1992, teaches the epoxidation of olefins using rhenium (VII) complexes in conjunction with hydrogen peroxide under near anhydrous conditions. The water in the hydrogen peroxide solution is removed by diluting the aqueous solution with tert-butyl alcohol, drying the solution over anhydrous magnesium sulfate, and removing the hydrated salt by filtration. The rhenium complex is then added to the alcoholic hydrogen peroxide solution followed by the addition of the olefin to carry out the epoxidation. The reaction must be conducted at relatively low temperatures, for example, -30.degree. C. to +10.degree. C., so that the oxidation leads selectively to the epoxide and further reaction to form the diol is suppressed. The required steps to provide the anhydrous conditions and the requirement of low temperatures under which the reaction is carried out make this procedure impractical on a commercial scale.
U.S. Pat. No. 5,166,372 issued on Nov. 24, 1992, describes the use of nitrogen containing heterocycles as ligands to rhenium catalysts used with hydrogen peroxide to epoxidize olefins. The reference claims that this class of organorhenium oxide catalysts tends to produce the lowest levels of undesired 1,2-diol side-products formed by hydrolysis. However, these compounds also modulate the activity of the catalysts downward, and thus slow the reaction rate considerably. The disclosed epoxidation method further employs a secondary alkyl aryl alcohol in combination with molecular oxygen to produce the hydrogen peroxide in situ. One of the byproducts of this reaction is the corresponding alkyl aryl ketone, which must then be hydrogenated over a platinum or palladium catalyst to convert it back to the alkyl aryl alcohol. In addition, the water content of the reaction mixture is sought to be maintained below four weight percent, and most preferably below one weight percent, by removing water formed during the oxidation from the reaction vessel with unreacted oxygen and inert gases. As can be understood, this technology requires specialized equipment that makes the method commercially unattractive.